Hi Mike
Welcome to MINI2 and sorry noone has come up with any suggestions
The trouble with problems like this is it could be a simple as a door switch, as you suggest, but it could equally be someting nasty like a computer fault. As I understand it, the MINI has two computers; one for engine management and one (called the Body Control Unit) to control other things like lights, windows etc.
If its a computer fault then the dealer should be able to detect this by reading the diagnostics. They should be able to tell whether its a big or small job to fix.
Best thing is to let the dealer have a look and let you know how much it would cost to fix.
Please report back and let us know how you get on.
Good luck.
Tony